This specification relates to data processing and content distribution.
The Internet provides access to a wide variety of resources. For example, video and/or audio files, as well as web pages for particular subjects or that present particular news articles are accessible over the Internet. To identify resources that may satisfy a user's informational need, the user can submit a search query to a search system and receive a search results page that identifies one or more resources. The search results page can include “slots” (i.e., specified portions of the web page) in which advertisements (or other content items) can be presented. Advertisements or other content items that are presented in the slots are selected for presentation by a content distribution system that can perform an auction as part of the selection process.